Method for polishing magnetic oxide materials

ABSTRACT

This invention relates to a method of polishing magnetic oxides or bubble crystal surfaces to a featureless and strain-free condition. The method comprises pre-polishing or lapping with a suspension of polycrystalline garnets to a conchoidal condition followed by final polishing with a zirconium oxide slurry under polishing pressure between 2 and 40 psi.

United States Patent Mendel [451 May 16, 1972 [54] METHOD FOR POLISHINGMAGNETIC OXIDE MATERIALS [72] Inventor: Eric Mendel, Poughkeepsie, NY.

[73] Assignee: International Business Machines Corporation, Armonk, NY.

[22] Filed: Jan. 28, 1971 [21] Appl.No.: 110,780

3,062,745 1 H1962 Gaynor et al. .....252/56 3,328,141 6/1967 Lachapelle..51/307 3,388,508 6/1968 Sieger et a]. l/284 3,453,784 7/1969 Highberget al. ..5 H283 3,460,295 8/1969 Teeter ..s1/2s3 OTHER PUBLlCATIONS BellSwitches Bubble Material" by Martin Gold, Electronic News, Apr. 27,1970.

Chemical Polish for Rare Earth Orthoferrites, by L. K. Shick,Electrochemical Society, Vol. 1 18, No. 1, January, l97l,pages 179-181.

Properties and Device Applications of Magnetic Domains in Orthoferritesby A. H. Bobeck, The Bell System Technical Journal, October 1967,pages1901- 1925.

Primary Examiner-Lester M. Swingle AttorneyHanifin and .lancin andDaniel E. [go

[5 7] ABSTRACT This invention relates to a method of polishing magneticoxides or bubble crystal surfaces to a featureless and strain-freecondition. The method comprises pre-polishing or lapping with asuspension of polycrystalline garnets to a conchoidal condition followedby final polishing with a zirconium oxide slurryunder polishing pressurebetween 2 and 40 psi.

9 Claims, No Drawings METHOD FOR POLISHING MAGNETIC OXIDE MATERIALSBACKGROUND OF THE INVENTION The surface treatment and polishing ofmagnetic host materials commonly referred to as orthoferrites, garnets,and magnetic oxides or bubbles requires methods and techniques differentfrom the procedures heretofore followed in the polishing of metals,ceramics and other crystalline materials. Application Ser. No. 1 10,779,filed Jan. 28, 1971, and entitled Method For Polishing Magnetic OxideMaterials discloses a silicon dioxide method of polishing magneticoxides. The orthoferrites are a class of rare earth (RE), iron oxideshaving the general formula REFeO and they have a perovskite relatedorthorombic structure. The iron in these compounds is trivalent incontrast to the spinel ferrites where both divalent and trivalent ironexists. These magnetic materials are meltgrown and growth techniques canbe grouped into three classes; namely, pure melts, self-fluxes meltswhere an excess of one crystal constituent serves as a solvent, andmolten solutions employing an added solvent. Similarly, the Czachrasskimolten pool single crystal pulling method is applicable to theproduction of magnetic oxide materials. These techniques produce amonocrystalline structure capable of containing magnetic domains orbubbles. It is also known to grow homogeneous uniaxial magnetic garnetfilms using liquid phase epitaxial techniques. These techniques permitfabrication of devices with an excess of 1 million bubbles per squareinch for use in computer and digital communications applications. Inorder to accomplish this type film growth, the substrate surface must bepolished perfectly to a featureless state free of cracks and strains.Otherwise, the film growth will be imperfect. Before bubbles can becreated in a crystal, a source or host crystal for the bubbles mustfirst be grown. A rare earth such as thulium or terbium is placed in acrucible and a ferrite is added. This combination is then heated to wellabove the molten state and then cooled slowly to room temperature. Theresultant single crystal magnetic oxide is called an orthoferrite. Thecrystal is then sliced and polished to make a platelet several milsthick. The condition of the surface of single crystal magnetic materialsis important, because substantial narrowing of the ferromagneticresonance line width may be accomplished if the surface of a specimen isprepared in a featureless, highly polished, and strain-free state.Strain-free domain patterns are more readily made if the strainedsurface layers caused by mechanical polishing or other treatments can beremoved.

DESCRIPTION OF THE PRIOR ART It has been found that certainferromagnetic oxides become optically transparent when their thicknessis reduced to several tens of microns. These materials exhibit Faradayrotation and magnetic birefringence in transmitted polarized light.These effects have been utilized to study the magnetic domain structurein the certain iron garnet oxides. The surfaces of these materials havebeen mechanically polished or chemically treated during the polishingoperation to produce polished surfaces on both sides of a specimen.

The prior art methods used in an attempt to prepare highly polishedfeatureless surfaces on the subject material specimens have their originin either the metallographic or petrographic art. The metallographicpractices generally entail sampling of the material by sawing or cuttingoff a representative section, rough-grinding, fine-grinding, polishing,and removal of the damaged surface layers by etching, chemical orelectro-chemical polishing. The rough-grinding and fine-grinding stepsmentioned above utilize successively finer grades of silicon carbide oremery abrasive papers. Polishing is accomplished with successively finergrades of diamond grit, aluminum oxide, or magnesium oxide abrasivepowders.

The petrographic methods have been reasonably standardized and aregenerally applied in the study and utilization of refractories,ceramics, and gem stones. Often, refractory and ceramic specimens areprepared as thin sections. The preparation of these materials follow thesame procedure and sequence of steps as those mentioned above andapplicable to metallographic specimen preparation, except that there isa greater tendency to use silicon carbide and diamond in lapping andpolishing down, because these specimens are harder than metals.Similarly, the lap and polish surfaces differ greatly as compared tocloths used in metallographic polishing. The final thickness of thesespecimens is often important and the mounting of the specimen istherefore more critical than the degree of sophistication used in thesurface treatment operations. Flatness and thickness of the specimen isimportant in order to produce a substrate of uniform thickness andparallel surfaces. Chemical mechanical methods have also provedinadequate and impractical. The use of hot phosphoric acid attemperatures of approaching 500 C to chemically polish the subjectmaterials by rotating a specimen in the hot I-I PO bath. 4

These methods have failed to produce the required specular anddamage-free surfaces on flux-grown or other type single crystal garnets,orthoferrites and similar materials.

Cracks, holes, surface-scratches, twinning planes and inclusions orother planes are believed to be caused by the prior art polishing andsurface treatment methods all of which impede domain wall motion andprevent control of the establishment of domains into singleconfigurations. These conditions also restrict narrowing of theferromagnetic resonance line width which can be accomplished if thesurface of the specimen is prepared in a highly polished and strain-freestate. Elimination of mechanical polishing defects and strains makesobservations of strain-free domain patterns more readily possible.Damaged surface conditions on thin platelets of the basic materials ofthis invention have been removed to some extent by chemical polishingand etching. However, these chemical dissolution techniques cannot beapplied to all materials. Attempts to eliminate strains and internalcrystal dislocations by high temperature annealing in purified oxygenand argon atmospheres have been reported, where the orthoferrite thinplatelets are positioned in an orthoferrite holder within an appropriateannealing furnace and atmosphere.

SUMMARY OF THE INVENTION It is an object of this invention to provide amethod for the surface treatment of single crystal magnetic materialswhereby the polished surface is featureless and specular and damagefree.

It is a further object of this invention to provide a method forpolishing single crystal garnets and orthoferrites having featurelesssurfaces and void of internal sub-surface strains.

A still further object of this invention is to provide a method for thesurface treatment of magnetic monocrystalline materials to producecompletely featureless surfaces void of cracks, holes, scratches,preferential topography and the like.

It is still a further object of this invention to provide a surfacetreatment or polishing method for accomplishing the aforesaid resultswithin practical time limits and suitable for large-scale commercialmanufacturing procedures.

A still further object of this invention is to provide a method forpolishing and surface treatment of monocrystalline magnetic materials soas to produce a slice or platelet free of internal strains and strainedsurface layers.

These and other objects are accomplished in accordance with the broadaspects of the present invention by providing a pre-polishing or lappingstep using a suspended mineral silicate and which produces a shallowdish-like flaked conchoidal fractured surface, followed by a polishingstep utilizing materials and conditions which produce a completelypolished featureless strain-free specimen.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of the preferred embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to avoid and minimizethe penetration of damage into the crystal during the polishing steps ofthis invention, an abrasive material which breaks down rather rapidlyand removes stock as a result of conchoidal fracturing is contemplated,for example, in the use of natural polycrystalline garnet abrasivepowders which is believed to remove material chips or flakes from thesurface of a specimen through the exertion of lateral rather thanvertical forces. Garnets are a group of silicate minerals with thegeneral formula R;," R,' (SiO is calcium, magnesium, iron or manganeseand R' is aluminum, iron titanium or chromium. Conchoidal fracturelapping produces a very thin layer of damaged material on the surface ofthe workpiece by removal or relatively uniform shallow-dished or flakedpieces. In contrast, lapped surfaces can be obtained where the abrasivegrains produce a series of deep penetrating flaws that extend into thebulk of the workpiece in a non-uniform manner.

When the damaged (lapped) layer is very thin and uniform in thickness,as results in conchoidal flaking, two conditions occur separately or actsimultaneously. The abrasive grains break down rapidly because they arefriable and not harder than the workpiece on the forces acting to removematerial from the surface are primarily tangential to the surface of thematerial and act to flake out shallow conchoidal stock or shear awayexisting protuberances. It is essential and critical that this type ofsurface is accomplished in the lapping or prepolishing step.

The gentlest or least severe practical lapping system where stockremoval occurs by conchoidal fracturing is the use of polycrystallinegarnet powder having a a particle size between 7 and 30 microns with apearlitically cast-iron wheel system. Frosted-plate glass can besubstituted for the cast-iron wheel. Yttrium iron garnet, Y Fe -,O,crystal was sliced into platelets 20 mils thick. These platelets werethen cemented onto a steel plug with glycol phthalate. A conventionalaqueous lapping slurry was prepared with 12 micron-sized garnet powdersand water. The distribution pattern of the 12 micron-sized garnet powdershould be between and 14 microns. The garnet slurry was applied tofrosted-glass plate and sliced specimens of garnet affixed to the steelplugs were manually moved over the glass plate in a random figure-eighttype motion. The stock removal was extremely rapid and a uniform surfacewith two to three mil conchoidal stock removal was accomplished afterten minutes of pre-polishing. A comparable diamond prepolished sequencerequires two or three hours or more polishing time and produces asurface having fissures and cracks and internal strains which makesfinal featureless polishing unobtainable. The aforesaid lapping orpre-polishing step was followed by final polishing steps. Although agarnet-type abrasive, suspended in water, is adequate for accomplishingthe pre-polishing or lapping step, the water-based slurry tends toseparate and settle upon standing. A wide variety of commercialsuspending agents are available for slurry preparation to maintain theabrasive material in constant suspension.

The pre-polishing surface preparation described above reduces processpolishing time from 1 week to not more than 4 hours for a specimenpolished on both sides to a thickness of l to 2 mils and possessing afeatureless damage-free surface.

Garnet grit ranging in particle size from three to thirty microinches isuseful. Particle sizes of about 12 microinches are believed to beoptimum.

Final polishing is accomplished by using a conventional water polishingslurry of zirconium oxide powders. An aqueous suspension equivalent tobetween 150 to 700 grams SiO, in 3 to 4 liters of water produces asmooth specular and featureless surface. Surface conditions are measuredand judged by visual naked eye and microscopic observations, as well asstandard examination of control specimens after hot H PO,

etching procedure.

Final polishing is accomplished by using conventional polishing wheelequipment and a suitable polishing cloth in conjunction with thepolishing slurr Any convenient polishmg wheel speed for manual or mecanical manipulation can be used. A wheel speed between 60 andrevolutions per minute with a presence of from 10 to 40 psi isillustrative of contemplated process conditions.

Upon complete polishing of the first surface, the process is repeatedfor the second or opposing surface. Simultaneous mechanical doublesurface polishing is contemplated within the scope of this method usingappropriate handling techniques and thickness measurement methods suchas the air gauge of microscopic differential focusing. Final featurelessinspection and control is accomplished by the use of Nomanskiillumination, which is a form of interference contrast lighting todetect and see a defective structure.

While the invention has been particularly described and shown withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:

1. A method for polishing magnetic crystalline material comprising:

lapping the surface with a suspension of polycrystalline silicatemineral, and

finish polishing with a suspension of zirconium oxide.

2. A method in accordance with claim 1 wherein said silicate material isgarnet.

3. A method in accordance with claim 1 wherein the particle size of saidpolycrystalline silicate is between 7 and 30 microns.

4. A method in accordance with claim 1 wherein said zirconium oxidesuspension is aqueous.

5. A method in accordance with claim 1 wherein said finish polishing isdone under pressure between 10 and 40 psi.

6. A method for polishing magnetic crystalline material comprising:

lapping the surface with an aqueous suspension of polycrystallinesilicate mineral to produce a surface characterized by shallow flakedconchoidal fracture patterns, and

finish polishing with an aqueous suspension of zirconium oxide at apressure between 10 and 40 psi.

7. A method in accordance with claim 6 wherein said polycrystallinesilicate material is garnet.

8. A method in accordance with claim 6 wherein said polycrystallinesilicate material has a predominant particle size of 12 microns.

9. A method for polishing magnetic crystalline material which comprises:

lapping the surface with an aqueous suspension of garnet having apredominant particle size of 12 microns to produce a surfacecharacterized by shallow flaked conchoidal fracture patterns, and

finish polishing said surfaces with an aqueous slurry of zirconium oxideunder a pressure between 2 and 40 psi.

2. A method in accordance with claim 1 wherein said silicate material isgarnet.
 3. A method in accordance with claim 1 wherein the particle sizeof said polycrystalline silicate is between 7 and 30 microns.
 4. Amethod in accordance with claim 1 wherein said zirconium oxidesuspension is aqueous.
 5. A method in accordance with claim 1 whereinsaid finish polishing is done under pressure between 10 and 40 psi.
 6. Amethod for polishing magnetic crystalLine material comprising: lappingthe surface with an aqueous suspension of polycrystalline silicatemineral to produce a surface characterized by shallow flaked conchoidalfracture patterns, and finish polishing with an aqueous suspension ofzirconium oxide at a pressure between 10 and 40 psi.
 7. A method inaccordance with claim 6 wherein said polycrystalline silicate materialis garnet.
 8. A method in accordance with claim 6 wherein saidpolycrystalline silicate material has a predominant particle size of 12microns.
 9. A method for polishing magnetic crystalline material whichcomprises: lapping the surface with an aqueous suspension of garnethaving a predominant particle size of 12 microns to produce a surfacecharacterized by shallow flaked conchoidal fracture patterns, and finishpolishing said surfaces with an aqueous slurry of zirconium oxide undera pressure between 2 and 40 psi.